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GeekInterview.com > Interview Questions > Oracle > SQL
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| Question: What is difference between UNIQUE and PRIMARY KEY constraints?
Answer:
A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL. |
| May 05, 2007 07:54:26 |
#5 |
| amit |
Member Since: Visitor Total Comments: N/A |
RE: What is difference between UNIQUE and PRIMARY KEY ...
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| Primary key creates clustered index whereas unique key creates non clustered index. Primary key won't allow NULL values whereas unique key allows only one NULL values. |
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